Hoists have been widely used to hoist workmen and materials on building construction work sites. A typical hoist includes a hoisting tower constructed of lattices of steel angle iron, and a lift cage or platform equipped to be vertically movable along the hoisting tower. The lift cage is guided by rollers along a vertical path provided in the hoisting tower and is driven by motors having pinions engaging with vertical racks attached to the tower along the vertical path.
In such a rack and pinion drive system, there are usually provided safety brakes to stop a cage when it exceeds a predetermined speed. A particular safety brake generally has an input pinion gear in engagement with the rack. In the event that the input gear rotates faster than a predetermined rate, the safety brake starts to operate stopping its input gear to provide a braking force to the cage.
U.S. Pat. No. 4,258,832, to Thorp, discloses a safety brake in which a centrifugal control assembly is connected to an input pinion gear engaging a vertical rack of a hoist, and a dog of the control assembly is positioned inside a drum of a brake assembly. When the dog is thrown outwardly by centrifugal force, it engages an internal tooth of the drum and simultaneously a tab of an actuator so that the drum and the actuator are both operatively connected to the input pinion gear for rotation. Movable braking disks are attached to the drum for rotation therewith. The movable braking disks are interleaved with the fixed braking disks which are non-rotatably but slidably fixed to a housing of the safety brake. The actuator rotates a ball screw and a nut of the ball screw moves axially to compress slidably fixed braking disks against the rotating braking disks to effect a braking force until the cage is stopped.
Although this patent apparatus presented an advance in the art, it was not without any drawbacks. Since the movable disks are attached to the drum, they do not rotate until the drum is operatively connected to the main shaft. Those movable disks are therefore required to be lubricated. Such lubrication disadvantageously acts to reduce friction between the movable and fixed disks when compressed to each other, which friction produces a braking force.
The present invention provides a further advancement in the art, as will be fully explained below.